The process of rotational molding, also called rotomolding, of plastic has been known since the 1940s for the preparation of hollow plastic articles.
The rotomolding process consists of adding a thermoplastic polymer into a mold, rotating the mold so that all the points of the internal surface of the mold are in contact with the polymer while heating the mold, so as to deposit the aforementioned molten polymer on the internal surface of the mold. Thereafter, a stage of cooling allows the solidification of the plastic article, which is then removed from the mold.
Rotational molding is advantageous because it avoids applying stress and strain to the plastic, which generally occurs in other transformations, for example in injection molding. Indeed, the plastic does not undergo malaxation or compaction as in an extruder or in injection molding. Rotational molding is particularly suitable for preparing large-sized articles, such as furniture, tanks, drums, reservoirs etc.
Polyethylene represents more than 80% of the polymers used in the rotomolding markets. This is due to the outstanding resistance of polyethylene to thermal degradation during processing, to its easy grinding, good flowability and low temperature impact properties.
Rotomolding is used for the manufacture of simple to complex, hollow plastic products. A variety of materials can be used such as polyethylene, polycarbonate polyamide or polyvinylchloride (PVC). For example, polyethylene compositions are used for the production of a wide variety of articles. Particularly, polyethylene drums and tanks can easily be manufactured by rotomolding. Often these drums and tanks are exposed to numerous stresses during their lifetime, and that exposure may result in cracks or breaks. Thus, there is an ongoing need to develop rotomolded articles that display resistance to the development of cracks or breaks.
It is therefore an object of the present invention to provide rotomolded articles comprising polyethylene resin having improved mechanical properties and improved processability.